Overview of open cut coal mining, Hunter Coalfield – A geotechnical perspective

An increased number of proposed multi-storey buildings in Newcastle and Charlestown has heightened concerns regarding the long-term stability of underlying abandoned bord and pillar workings. The workings are approximately 100 years old, often irregular in layout and are now inundated with water. The last large scale pillar crush occurred beneath Newcastle in 1908. The lack of subsequent crushes demonstrates stability since abandonment. However, possible deterioration of pillar, floor and roof material due to the effects of time and water may affect future stability. A proposed eight storey development in Newcastle West required the assessment of abandoned first and nearby second workings in the Borehole Seam at approximately 64 m depth. Three dimensional numerical modelling using the finite difference program FLAC-3D was used to assess pillar stability and subsidence over time with consideration of first workings, secondary pillar extraction and inundation of the workings. The paper presents a description of the workings, investigation work and the numerical modelling methodology and results. The pillar stresses are compared to estimates made using empirical methods for various stages of mining.